The present invention relates to a control apparatus for a power steering system for producing a proper steering force in accordance with a vehicle velocity and a steering angle.
A steering force of a vehicle is generally increased at a low velocity. In a conventional power steering system for producing auxiliary power in accordance with a current steering force, only a small steering force is required at a low vehicle velocity, but a steering force at a high vehicle velocity is excessively small, thus leading to unstable driving. Various power steering systems have been proposed to decrease a ratio of an auxiliary force input to its output when a vehicle velocity is increased so as to solve the above problem. By using such a power steering system, proper steering forces are required at high and low velocity regions respectively. In other words, a relatively small steering force is required at a low vehicle velocity, and a relatively large steering force is required at a high vehicle velocity.
In general, a steering force is preferably increased when a steering angle is increased. The more the driver turns the steering wheel, the more the steering force preferably increases for safe driving as well as for good steering feelings. This tendency is typical in high-speed driving. However, in a conventional power steering system, an increase in steering force upon turning of the steering wheel is small at a high vehicle velocity. Demand in this respect has arisen for safe driving and good driving feelings. In particular, a driver may overturn the steering wheel due to a small steering force required at a high vehicle velocity, which may lead to a disastrous accident. Therefore, in order to prevent this, in a conventional power steering system, a relatively large steering force is employed throughout the entire velocity range. However, such a large steering force does not guarantee comfortable driving. In particular, long touring at a high speed causes fatigue of the driver.